Embodiments of the disclosure relate generally to the fabrication of integrated circuit (IC) structures for bipolar transistors. More specifically, embodiments of the present disclosure provide processes of forming base materials for bipolar transistors (also known as a bipolar junction transistors, “BJTs,” or heterojunction bipolar transistors, “HBTs”) with different physical and electrical characteristics on a single substrate.
In integrated circuit (IC) structures, a transistor is a critical component for implementing proposed circuitry designs. In analog circuitry, numerous functions can be implemented with bipolar transistors. For example, the ability to control the flow of electric current between terminals of the transistor can allow the transistor to act as a switch, and therefore act as a building block for logic functions. Bipolar junction transistors are found in a wide variety of devices, e.g., radiofrequency transceivers for wireless networking and cellular applications, multi-gigabit analog-to-digital converters, optical networks, automotive radar, and high-speed circuits. Generally, a bipolar transistor includes three electrical terminals: a collector, a base, and an emitter. The base of the transistor can be subdivided into an intrinsic base positioned directly between the collector and emitter, and an extrinsic base which connects to the base terminal where other circuit elements can connect to the base. The flow of electricity between the collector and emitter terminals of a bipolar transistor can be controlled by adjusting the electric current at the base terminal or by changing the voltage difference between the base and emitter terminals.
Bipolar transistors are a building block of many amplifier circuits, which can be designed to perform various electrical functions, e.g., modifying a current or voltage between two terminals. The processes used to form bipolar transistors can define various attributes of the transistors formed, e.g., base-emitter capacitance, base resistance, current gain across terminals, etc. In some cases, a fabricator may wish to form multiple bipolar transistors with different electrical properties on the same wafer due to the expectation that some bipolar transistors will be used for different applications, e.g., different types of amplifier circuits. This concern presents a significant technical challenge because multiple fabrication techniques may need to be combined or substituted for simpler fabrication techniques to yield multiple types of transistors on a single wafer. Conventional approaches may require multiple wafers to be used when forming transistors used in different types of amplifier circuits.